How to alter the speed in Dwarf Fortress?

Poindexter is right, you can't directly. Certain tricks like caging all pets or limiting the population of your fort might help the game run faster with the hardware you've got. Even the layout of your fort can effect run time (because path-finding is a major CPU task). See the answer provided here: Extremely low FPS in Dwarf Fortress

But I would only go down those routes if the game is so slow as to be unplayable.

Things you can do to decrease speed:

• Start your fort near the ocean
• Have a larger embark area
• Increase your population as much as possible and free any caged animals.

Things you can do to increase speed:

• Start your fort somewhere without ocean or water
• Choose a small embark area
• Slaughter and/or cage all non-essential animals

Finally the speed of your computer will certainly affect game speed. I occasionally will start a fort on my old laptop so that the game runs a little slower at the start, and then move the save to my PC once things get going and the framerate slows a bit.

Yes, you can try.

Lowering [FPS_CAP:X] in the datainitinit.ini file can lower the speed of the game. If you're running at your FPS cap, increasing it can increase the speed of the game. Normally, however, the cap is not reached and your processor speed is the bottleneck.

Lowering [G_FPS_CAP:X] can actually increase the speed slightly, if you're not running at your FPS cap.

Lastly, I will add that the single most important thing I have personally found that decreases speed is your embark area. If it's large, your game will start to crawl after you get a larger number of dwarfs. And now that magma can now be found in every embark square, I usually stick to areas of 3x3, and my FPS hums along at a nice 120.

Above answers are answers to question "Can I make DF run faster", which is a bit different question from "How can one increase/decrease the speed of Dwarf Fortress?"

You can manipulate simulation time by changing FPS limit. By default, you can do that using Alt+equals, Alt-mins key binding. The upper speed limit is bound to your CPU processing capabilities.